Lemma 27.2.5.label Let $G$ be a locally compact group, $f, f'\in C_{c}^{+}(G)$, and $\eps > 0$, then there exists $V \in \cn_{G}(1)$ such that for any $g \in C_{c}^{+}(V)$ with $g \ne 0$,

\[(f: g) + (f': g) \le (f + f': g) + \eps\]

Proof, [Lemma 2.18, Fol16]. By Urysohn’s Lemma, there exists $\eta \in C_{c}^{+}(G; [0, 1])$ such that $\eta|_{\supp{f} \cup \supp{f'}}= 1$.

Let $\delta > 0$, and define

\[H = f + f' + \delta \eta \quad h = \frac{f}{H}\quad h' = \frac{f'}{H}\]

By Proposition 27.1.2, there exists $V \in \cn_{G}(1)$ such that for any $x, y \in G$ with $x^{-1}y \in V$,

\[|h(x) - h(y)|, |h'(x) - h'(y)| < \delta\]

Let $g \in C_{c}^{+}(V)$, $\seqf{c_j}\subset [0, \infty)$, and $\seqf{x_j}\subset G$ such that $H \le \sum_{j = 1}^{n} c_{j} L_{x_j}\phi$, then for each $x \in G$,

\begin{align*}f(x)&= H(x)h(x) \le \sum_{j = 1}^{n} c_{j} L_{x_j}g(x)h(x) = \sum_{j = 1}^{n} c_{j}g(x_{j}^{-1}x)h(x) \\&\le \sum_{j = 1}^{n} c_{j}[h(x_{j}) + \delta] \cdot L_{x_j}g(x)\end{align*}

Likewise,

\[f'(x) \le \sum_{j = 1}^{n} c_{j}[h'(x_{j}) + \delta] \cdot L_{x_j}g(x)\]

As $h + h' \le 1$,

\[(f: g) + (f': g) \le \sum_{j = 1}^{n} c_{j}[h(x_{j}) + h'(x_{j}) + 2\delta]\]

Since the above holds for all such $\seqf{c_j}\subset [0, \infty)$ and $\seqf{x_j}\subset G$,

\begin{align*}(f: g) + (f': g)&\le (1 + 2\delta)(H: g) \\&\le (1 + 2\delta)[(f + f': g) + \delta(\eta: g)]\end{align*}

$\square$

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